1. Field of the Invention
Aspects of the present invention relate to a lithium ion secondary battery, and more particularly to an electrode assembly for a lithium ion secondary battery having ceramic separation films and a typical isolation film.
2. Description of the Related Art
A secondary battery is a rechargeable battery which has such a high capability that its volume can be reduced and its capacity can be increased. Recently, as demand for portable electronic devices, such as camcorders, portable computers, portable phones, and the like, has increased, various secondary batteries, used as electric power sources for these portable electronic devices, have been researched and developed. Such representative secondary batteries which have been developed and used include nickel hydrogen batteries, lithium ion batteries, lithium ion polymer batteries, and the like.
Lithium, used as a material in some secondary batteries, is suitable for making batteries having a large capacity per unit mass, because among other reasons, the Li atoms are small and mobile. On the other hand, since the lithium reacts with water, anhydrous electrolytes are used for lithium based-batteries. In this case, since the lithium is not subjected to an effect of voltage due to the electric resolution of water, the lithium based-batteries can generate electromotive forces to the extent of 3 to 4 volts.
The anhydrous electrolyte used for the lithium ion secondary battery generally includes a liquid phase electrolyte and a solid phase electrolyte. The liquid phase electrolyte is obtained by dissociating a lithium salt in an organic solvent. Carbonate containing alkyl groups such as ethylene carbonate and propylene carbonate or organic compounds similar to the carbonates are used as the organic solvent.
However, the ion conductance of the electrolyte is low in the lithium ion secondary battery. The lower ion conductance of the electrolyte can be overcome by expanding an area of an active material on each of the electrodes and enlarging the opposing surfaces of two electrodes.
The enlargement of opposing surfaces of the electrodes is limited by various factors. In the end, the lower ion conductance of the electrolyte allows the impedance in the battery to increase, thereby causing a greater drop in the inner voltage. Specifically, the lower ion conductance of the electrolyte serves as a primary factor to restrain electric current of the lithium ion secondary battery, thereby limiting the output of the lithium ion secondary battery, even when large current discharge is required.
In addition, a separator also is a primary factor in restricting the movement of the lithium ions between the two electrodes. If the separator existing between the two electrodes does not have enough transmittance and wettability for the electrolyte, the separator restricts the movement of the lithium ions between the two electrodes, so that the electric characteristic of the lithium secondary battery is degraded.
Therefore, important characteristics of the separator relating to the performance of the lithium secondary battery include the heat resistance, the resistance to heat distortion, the chemical resistance, the mechanical strength of the separator, a separator aperture, (a cavity area per unit volume or a porosity in a certain section of the separator), and the wettability to the electrolyte.
Furthermore, the separator of the lithium ion battery plays a role as a safety device for preventing the lithium secondary battery from overheating. The polyolefin based, finely porous film, usually used as a material for the separator, is softened and partially melted when the temperature of the lithium battery rises over a predetermined point due to a malfunction of the lithium battery whereupon the fine apertures of the porous film which serve as passage ways for the electrolyte and the lithium ions are squeezed shut. The movement of the lithium ions is thus interrupted and the flow of the electric current in/outside the lithium secondary battery is stopped, so that the increase in temperature of the lithium secondary battery caused by the electric current is halted.
Where the temperature of the lithium secondary battery suddenly rises because of a certain reason, i.e. external thermal transition, etc., and the increase of the temperature of the lithium secondary battery continues for a certain time even though the fine apertures of the separator are sealed, then the separator can be damaged. Specifically, a part of the separator is melted, at which point the two electrodes of the lithium secondary battery may be in direct contact with each other so as to cause an internal short circuiting. Further, the separator may shrink, thereby allowing the two electrodes to make contact with each other and cause further short circuiting. This short circuiting in the lithium secondary battery is very damaging to the lithium secondary battery and dangerous.
In addition, as the lithium secondary battery has a large capacity, a great amount of electric current can flow in the lithium secondary battery in a short time. In this case, when over-current flows in the lithium secondary battery, additional heat is generated that does not dissipate easily contributing to continued melting of the separator, even though the fine apertures of the separator are closed so as to interrupt the flow of the electric current through that path. So there is an increasing possibility of damaging the separator to cause or exacerbate the short circuiting in the lithium secondary battery.
In this circumstance, although it is important to shut the apertures of the separator, it is more important to prevent the melting or shrinking of the separator when the lithium secondary battery is overheated. Specifically, to prevent the internal short circuiting between the electrodes due to an unstable separator even at a relatively high temperature (more than 200° C.), a ceramic separator, which is a porous film made of a mixture of particles of ceramic filler and binding agent, can be used as the separator.
FIG. 1 is a perspective view showing a conventional electrode assembly to which a ceramic film playing the role of a separator is applied. The electrode assembly 100 to which the conventional porous separation film is applied, includes an anode electrode plate 110, a cathode electrode plate 120, and a ceramic film 130. The anode electrode plate 110 is formed by coating a desired region of an anode current collector with an anode active material layer, while the cathode electrode plate 120 is formed by coating a desired region of a cathode current collector with a cathode active material layer. The ceramic film 130 is coated on the anode and cathode electrode plates 110 and 120, so as to prevent a short circuiting between the anode and cathode electrode plates 110 and 120 and to play the role of a separator to allow only lithium ions to move. The electrode assembly 100 is wound in a jelly-roll shape.
Lithium oxides such as LiCoO2, LiMn2O4, LiNiO2, and LiMnO2 are used as the anode active material. Further, carbon based-materials, Si, Sn, tin oxide, composite tin alloys, and transition metal oxides are used as the cathode active material.
The anode current collector of the anode electrode plate 110 is made of aluminum material. The cathode current collector of the cathode electrode plate 120 is made of copper. The separator 130 is usually made in such a manner that ceramic particles are uniformly dispersed in the mixture of binder and solvent so as to form a solution for a porous film and then the electrode plate which is formed by coating a current collector with the active material is dipped in the solution for the porous film. Zirconium oxide, alumina, silica, and mixtures thereof are used as the ceramic material.
In the case of a polygonal battery to which the separator is applied, exfoliation of the ceramic separator occurs at bent portions of the leading edge of the electrode plate in the jelly-roll type electrode assembly, thereby readily causing a short circuit. In addition, in the case of the polygonal or cylindrical battery to which the ceramic separator is applied, since the exfoliation of the ceramic separator may occur at both ends of the electrode plate at which an uncoated portion is formed, there is the danger of causing a short circuit.
In the case of the battery to which the conventional ceramic separator is applied, in order to prevent the occurrence of the short circuiting, a polyethylene film or a polypropylene film (hereinafter, referred to as the existing separation film) has been inserted between boundary surfaces of the anode and cathode electrodes. As a result, the electrode assembly becomes thick. Therefore, the length of the electrode plate must be reduced. Moreover, the resistance of the electrode plate increases and makes it difficult for the lithium ions to move smoothly.